Proper treatment of spinal injuries such as trauma, fractures, non-unions, tumors, cysts, and degenerated discs may involve filling a cavity that has been created by the pathology itself or by the action of a surgeon. Often the cavities are compressed, and require that the surfaces of the cavity be distracted from one another and then supported to return the spinal structure to its anatomic position and form. Furthermore, because spinal tissues such as vertebra and cartilage have structural and support roles in the body, it is critical that such cavities be repaired to allow reliable strength and support.
Compression fractures are one type of hard tissue injury belonging to a class of conditions that may be treated using devices and methods for separating, distracting, and supporting a fractured bone. For example, vertebral compression fractures are crushing injuries to one or more vertebra. A vertebral compression injury may be the result of a trauma to the spine, an underlying medical condition, or a combination of a trauma and an underlying condition. Osteoporosis and metastatic cancers are common medical conditions that also contribute to vertebral compression fractures because they weaken spinal bone, predisposing it to compressive injury.
Osteoporosis is a degenerative disease that reduces bone density, and makes bone more prone to fractures such as compression fractures. An osteoporosis-weakened bone can collapse during even normal activity. According to the National Institute of Health, vertebral compression fractures are the most common type of osteoporotic fractures.
Vertebral fractures may be painful and may deform the shape of the spine, resulting in unhealthy pressure on other parts of the body, loss of height, and changes in the body's center of gravity. Untreated, such changes and the resulting discomfort can become permanent, since the bone heals without expanding the compression.
Existing methods of treating bone injuries may involve highly invasive or inadequate treatments. For example, one method of treatment is percutaneous vertebroplasty. Vertebroplasty involves injecting bone filler (such as bone cement) into the collapsed vertebra to stabilize and strengthen the crushed bone. In vertebroplasty, physicians typically insert a small diameter guide wire or needle along the pedicle path intended for the bone filler delivery needle. The guide wire is advanced into the vertebral body under fluoroscopic guidance to the delivery point within the vertebrae. The access channel into the vertebra may be enlarged to accommodate the delivery tube. In some cases, the delivery tube is placed directly into a vertebral body and forms its own opening. In other cases, an access cannula is placed over the guide wire and advanced into the vertebral body. In both cases, a hollow needle or similar tube is placed into the vertebral body and used to deliver the bone filler into the vertebra.
When filling a bone cavity with bone filler using traditional vertebroplasty, fillers with lower viscosities may leak. Further, even fillers having low viscosities may require the application of a high pressure to disperse the bone filler throughout the vertebral body. However, application of high pressure also increases the risk of bone filler extravasation from the vertebral body. Conversely, injecting a bone filler having a higher viscosity may provide an even greater risk of “leaking” bone filler into sensitive adjacent body areas. Leaks or extrusion of the bone filler may be dangerous to a patient's health. For example, posterior extravasation from a vertebral body may cause spinal cord trauma, perhaps resulting in paralysis. Risk of leakage is even more acute when a bone filler is applied under pressure to expand a compression fracture, especially if the fracture has begun healing and requires substantial force to distract the cavity surfaces.
Furthermore, most bone cements and bone fillers are difficult to remove or to adjust. Removal and adjustment may be important when distracting a bone cavity. For example, removing a precise amount of bone filler may allow a surgeon to adjust the level of distraction of a vertebral compression fracture and correct the shape of the compressed bone. Many bone cements, once set, are difficult or impossible to remove without further, highly invasive, surgery. Even if the removal is attempted prior to the expiration of the setting time, the materials may have non-Newtonian flow characteristics requiring a substantial removal vacuum to achieve an initial and sudden movement.
The implants described herein may avoid many of the problems described above when filling a cavity within the body, and particularly a cavity within the spinal region. The use of segments contained within a flexible tube or sheath offers an alternative to packing or expanding a cavity within body tissue. This could be an advantage in the treatment cavities such as vertebral compression fractures since the use of a flexible tube reduces concerns of fluent material leakage from the internal vertebral space and provides more control in delivery. These devices may be used in other regions of the body where the filling of a cavity with stability and control is desired, and is not necessarily limited to the spinal region. For example, the devices described herein may be used to repair hip; tibia, and other areas of bone displacement.
In addition to traditional bone cements, a handful of other cavity filling materials have been suggested. In particular, biodegradable and/or bioabsorbable devices have been suggested. For example, U.S. Pat. No. 5,756,127 to Grisoni et al. describes a bioresorbable string of calcium sulfate hemihydrate (Plaster of Paris) beads and a means for producing these beads. However, the Grisoni device is not appropriate for spinal regions, and has many disadvantages. Calcium sulfate hemihydrate (Plaster of Paris) and similar materials have low elasticity and crush strength, making them unreliable as materials to distract and later support a spinal region, particularly during the early stages of the healing process. Filling materials that are readily compressed or crushed may shift within, or exit, the cavity altogether, leading to detrimental changes in the shape of the spinal region. Materials with low crush strength are poor choices in withstanding the stress of distracting spinal regions, and may be unable to maintain the distracted shape after filling a spinal region. Similar materials are the subjects of U.S. Pat. No. 6,579,533 to Tormala et al.
U.S. Pat. No. 5,702,454 to Baumgartner describes an implant made of an elastic plastic for implanting into an intervertebral disk. Because the Baumgartner implant is elastic and somewhat amorphic, it may be less effective for filling and distracting spinal cavities, particularly cavities benefiting from implants having some stiffness, such as non-soft tissue cavities, and cavities that benefit from a stable implant shape. This is particularly true where sustained distraction is desired.
U.S. Pat. No. 6,595,998 to Johnson et al. describes a tissue distraction device in which wafers are inserted to distract a tissue cavity by forming a wafer stack within the cavity. However, Johnson's column of wafers is not amenable to providing uniform support to all surfaces of a cavity, when such support is needed. For example, a tissue cavity supported or distracted on all sides of the cavity may be more stable.
U.S. Pat. No. 5,958,465 to Klemm et al. describes a method and apparatus for making drug-containing implants in the form of a string of beads comprising chains of small drug-containing plastic bodies arranged in series on a surgical wire or thread. Similar drug implanted beads-on-a-string are described in U.S. Pat. No. 6,183,768 to Harle and German Patents 2320373 to Klemm and 2651441 to Heusser. The Klemm, Harle, and Neusser implants are designed for drug delivery, and are embedded with one or more drugs which are released from the plastic (e.g. PMMA) beads (also called “corpuscles”). Thus, these implants may be limited in strength and durability because of the inclusion of a releasable drug, as well as the properties and shape of the implant beads.
In any event, none of the cited documents show the device and methods disclosed below. The devices described herein may address many of the problems identified above, particularly in the treatment of the spine.